Field of the Invention
The present invention relates to an image capturing apparatus, a method for controlling the same, an image processing apparatus, and an image processing method, and relates in particular to an image capturing apparatus, a method for controlling the same, an image processing apparatus, and an image processing method for adjusting the black level of an image signal captured using an image sensor.
Description of the Related Art
Conventionally, in some kinds of image capturing apparatuses that use a condensing optical system and an image sensor, a photoelectric conversion portion in each pixel portion provided below a condensing microlens that is formed on the image sensor is divided into a plurality of portions, and images obtained by dividing an exit pupil in the condensing optical system are acquired as image signals. Japanese Patent Laid-Open No. 2001-124984 discloses a technique for detecting focus based on image signals that include images obtained by dividing an exit pupil.
With the above-described configuration of conventional image sensors, image signals are independently acquired from a plurality of photoelectric conversion portions in every pixel portion, and therefore, a problem arises in that the number of pixels to be read out from an image sensor increases. For this reason, there is a proposed method in which signals are independently read out from a plurality of photoelectric conversion portions in only lines that belong to an area for detecting focus on the image sensor (focus detection area), whereas, in lines that do not belong to the focus detection area, signals from the plurality of photoelectric conversion portions are added together in the pixel portions and are then output. It is conceivable to thus reduce the number of pixels to be read out in the image sensor. This configuration enables focus detection to be performed using the image signals in the lines that are independently read out from among the image signals obtained from the image sensor, and also enables an image signal for recording to be generated by adding together the image signals.
However, employment of the above configuration causes a new problem. The signal level at the time when the amount of incident light is zero differs among the lines that have been read out due to a readout operation for the image signals from the image sensor, which may degrade the accuracy of black level adjustment processing and may further be a factor in the deterioration of image quality of the image signal to be recorded.